import React, {useState} from 'react'
import {useEffect} from 'react'

export default function App() {
  
  const [opacity, setOpacity] = useState(1)
  
  useEffect(() => {
    const opacityTimer = setTimeout(() => {
      opacity -= 0.1
      if (opacity <= 0) {
        opacity = 1
      }
      setOpacity(opacity)
    }, 100)
    return () => {
      clearInterval(opacityTimer)
    }
  }, [])



  return (
    <div>
      <h1 style={{opacity}}>ReactHook真好用</h1>
    </div>
  )
}
 